Immunology Practice Questions

Q: Immunoglobulin is used for all of the following diseases except?

Typhoid. **Explanation:** The correct answer is **Typhoid (Option C)**. Immunoglobulin therapy involves **Passive Immunization**, where pre-formed antibodies are administered to provide immediate, short-term protection. This is typically used for post-exposure prophylaxis or in patients who cannot mount an immune response. **Why Typhoid is the correct answer:** Typhoid fever is caused by *Salmonella typhi*. Management and prevention rely on **Active Immunization** (e.g., Injectable Vi antigen or Oral Ty21a vaccine) and antibiotics for treatment. There is no clinical indication for the use of human or animal-derived immunoglobulins in the management of Typhoid. **Analysis of other options:** * **Measles:** Human Normal Immunoglobulin (HNIG) is used for post-exposure prophylaxis in susceptible individuals (especially infants and pregnant women) if given within 6 days of exposure. * **Rabies:** Rabies Immunoglobulin (RIG) is a critical component of Category III bite management, providing immediate neutralizing antibodies at the wound site before the vaccine-induced antibodies appear. * **Chickenpox:** Varicella-Zoster Immunoglobulin (VZIG) is indicated for high-risk individuals (e.g., immunocompromised or neonates) exposed to the virus to prevent or attenuate the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Passive-Active Immunity:** In Rabies and Tetanus, both immunoglobulin (passive) and vaccine (active) are given simultaneously at different sites. * **Types of Immunoglobulins:** * *Specific (Hyperimmune):* Rabies (RIG), Hepatitis B (HBIG), Varicella (VZIG), Tetanus (TIG). * *Non-specific:* Human Normal Immunoglobulin (used for Measles and Hepatitis A). * **Contraindication:** Immunoglobulins should generally not be administered within 3 months of live attenuated vaccines (like MMR or Varicella) as they may interfere with the immune response.

Q: During the infancy of cardiac transplantation, nonhuman primate hearts were transplanted into humans to save lives. This type of transplantation is best described by which one of the following?

Xenograft: transplant from one species to another species. ### Explanation The correct answer is **D. Xenograft**. **1. Understanding the Correct Answer:** A **Xenograft** (or heterograft) refers to the transplantation of cells, tissues, or organs between members of **different species**. In the scenario described, the transfer of a heart from a nonhuman primate (e.g., baboon or chimpanzee) to a human represents a cross-species barrier. These grafts are highly immunogenic and typically trigger **hyperacute rejection** due to pre-existing natural antibodies (e.g., anti-Gal antibodies) against foreign surface antigens. **2. Analysis of Incorrect Options:** * **A. Allograft:** This is a transplant between genetically non-identical members of the **same species** (e.g., human to human). This is the most common type of clinical transplant. * **B. Autograft:** This involves moving tissue from one site to another on the **same individual** (e.g., a saphenous vein graft for CABG or a skin graft). There is no risk of immunological rejection. * **C. Isograft (Syngeneic graft):** This is a transplant between **genetically identical** individuals, such as monozygotic (identical) twins. Like autografts, these do not trigger an immune response. **3. NEET-PG Clinical Pearls:** * **Hyperacute Rejection:** Occurs within minutes to hours; mediated by **pre-formed antibodies** and Type II hypersensitivity. It is the primary barrier in Xenotransplantation. * **Acute Rejection:** Occurs within days to weeks; primarily mediated by **T-cells** (Type IV hypersensitivity). * **Chronic Rejection:** Occurs months to years later; characterized by fibrosis and **accelerated arteriosclerosis** (intimal thickening). * **Graft-versus-Host Disease (GVHD):** Occurs when immunocompetent T-cells in the graft attack the immunocompromised host (common in bone marrow transplants).

Q: Which complex of the complement system is known as the Membrane Attack Complex (MAC)?

C5-9. **Explanation:** The complement system is a vital component of innate immunity, consisting of a cascade of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens. **Why C5-9 is Correct:** The **Membrane Attack Complex (MAC)** is the final common pathway of all three complement activation routes (Classical, Alternative, and Lectin). It is formed by the sequential assembly of complement components **C5b, C6, C7, C8, and multiple C9 molecules**. This complex inserts itself into the lipid bilayer of the target cell membrane, forming a transmembrane pore. This leads to the loss of osmotic integrity, resulting in water influx and subsequent **osmotic lysis** of the pathogen. **Analysis of Incorrect Options:** * **C3a:** This is an **anaphylatoxin**. It triggers mast cell degranulation, leading to increased vascular permeability and smooth muscle contraction. * **C3b:** This is the primary **opsonin** of the complement system. It coats the surface of bacteria, making them more "tasty" for recognition and engulfment by phagocytes (via CR1 receptors). * **C4b:** This is a component of the C3 convertase in the classical and lectin pathways (C4b2a). While it helps propagate the cascade, it does not form the lytic pore. **High-Yield Clinical Pearls for NEET-PG:** * **Deficiency:** Patients with deficiencies in late complement components (**C5 to C9**) have a significantly increased susceptibility to recurrent infections by **Neisseria species** (Meningitidis and Gonorrhoeae). * **Potency:** C5a is the most potent anaphylatoxin and a strong chemoattractant for neutrophils. * **Regulation:** CD59 (Protectin) is a human cell surface protein that inhibits MAC formation, protecting host cells from "bystander" lysis. Its absence is seen in Paroxysmal Nocturnal Hemoglobinuria (PNH).

Q: Which of the following molecular phenomena in Ig genes is responsible for affinity maturation of antibody response?

Somatic hypermutation. **Explanation:** **Somatic Hypermutation (SHM)** is the correct answer. It is the process that occurs in the **germinal centers** of secondary lymphoid organs (lymph nodes and spleen) after an antigen challenge. During B-cell proliferation, point mutations occur at an extremely high rate in the **V (variable) region** genes of heavy and light chains. B-cells with mutations that result in a higher affinity for the antigen are preferentially selected to survive and differentiate into plasma cells. This iterative process is known as **Affinity Maturation**, ensuring that the secondary immune response is more effective than the primary. **Analysis of Incorrect Options:** * **Chain Shuffling:** This refers to the random pairing of different heavy and light chains. While it contributes to initial antibody diversity, it does not drive the refinement of affinity after antigen exposure. * **Junctional Diversity:** This occurs during **V(D)J recombination** in the bone marrow (antigen-independent phase). It involves the random addition or deletion of nucleotides (P and N nucleotides) at the junctions of gene segments. It creates initial diversity but not affinity maturation. * **Altered RNA Splicing:** This mechanism is responsible for the simultaneous expression of **IgM and IgD** on a mature B-cell or the switch from membrane-bound to secreted antibodies. It does not change the idiotype or affinity of the antibody. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme involved:** **AID (Activation-Induced Cytidine Deaminase)** is essential for both Somatic Hypermutation and Class Switch Recombination. * **Site:** Germinal centers of lymph nodes (specifically the **Dark Zone**). * **Timeline:** Occurs during the **secondary immune response** (IgG/IgA/IgE phase), not the primary IgM phase. * **Distinction:** V(D)J recombination = Antigen-independent (Bone marrow); SHM/Affinity Maturation = Antigen-dependent (Periphery).

Q: A 2-year-old boy presents with recurrent bacterial infections and Pneumocystis jiroveci pneumonia. Laboratory findings show markedly elevated IgM and low IgG. Which of the following is the most likely cause of his illness?

Deficiency in CD40 ligand or CD40. ### **Explanation** The clinical presentation of recurrent bacterial infections, *Pneumocystis jiroveci* pneumonia (PJP), and the characteristic laboratory profile (High IgM, Low IgG/IgA/IgE) is diagnostic of **Hyper-IgM Syndrome**. **1. Why the Correct Answer is Right:** Immunoglobulin class-switching (isotype switching) requires a "second signal" interaction between **CD40 ligand (CD154)** on activated T-cells and **CD40** on B-cells. Without this interaction, B-cells cannot switch from producing IgM to IgG, IgA, or IgE. * **CD40L deficiency** (X-linked) is the most common form. * The susceptibility to **PJP** (an opportunistic fungal infection) occurs because CD40-CD40L signaling is also essential for T-cell-mediated activation of macrophages. **2. Why the Other Options are Wrong:** * **Option A (STAT3 mutation):** Causes **Job Syndrome (Hyper-IgE Syndrome)**. It presents with the triad of Eczema, recurrent "cold" Staphylococcal abscesses, and high IgE levels, not high IgM. * **Option C (NADPH oxidase defect):** Causes **Chronic Granulomatous Disease (CGD)**. It presents with infections by catalase-positive organisms (e.g., *S. aureus*, *Aspergillus*) and abnormal Nitroblue Tetrazolium (NBT) or DHR tests. * **Option D (Defective cytokine signaling):** Refers to **SCID** (e.g., IL-2 receptor gamma chain deficiency). While it presents with PJP, it is characterized by the absence of T-cells and low levels of *all* immunoglobulin classes (including IgM). **3. NEET-PG High-Yield Pearls:** * **Inheritance:** Most common form is X-linked Recessive (CD40L deficiency). * **Classic Lab:** ↑ IgM, ↓↓ IgG, ↓↓ IgA, ↓↓ IgE. * **Key Association:** Increased risk of *Cryptosporidium* diarrhea and biliary tract disease. * **Distinction:** Unlike other B-cell defects, Hyper-IgM presents with opportunistic infections (PJP) due to the T-cell signaling defect.

Q: Which of the following components of the immune system is responsible for the secretion of lymphokines?

T lymphocytes. ### Explanation **1. Why T Lymphocytes are the Correct Answer:** Lymphokines are a subset of cytokines produced specifically by **lymphocytes**, primarily **T lymphocytes (T cells)**. When a T cell encounters an antigen presented by an MHC molecule, it becomes activated and secretes lymphokines (such as IL-2, IL-3, Interferon-gamma, and TNF-beta). These chemical messengers act as "mediators of cellular immunity," signaling other immune cells like macrophages and B cells to migrate to the site of infection and initiate an inflammatory response. **2. Analysis of Incorrect Options:** * **A. B Lymphocytes:** While B cells are lymphocytes, their primary role is **humoral immunity** through the production of **antibodies (immunoglobulins)**. Although they can secrete some cytokines, they are not the classic source of lymphokines. * **B. Eosinophils:** These are granulocytes primarily involved in allergic reactions and defense against **parasitic infections**. They release preformed mediators (like Major Basic Protein) rather than lymphokines. * **C. Macrophages:** Macrophages secrete **monokines** (e.g., IL-1, TNF-alpha). While both lymphokines and monokines are types of cytokines, the term "lymphokine" is strictly reserved for products of lymphocytes. **3. NEET-PG High-Yield Pearls:** * **Cytokine Nomenclature:** Cytokines from lymphocytes = **Lymphokines**; Cytokines from monocytes/macrophages = **Monokines**. * **Th1 vs. Th2:** Th1 cells secrete **IFN-γ and IL-2** (cell-mediated immunity), while Th2 cells secrete **IL-4, IL-5, and IL-13** (humoral/allergic response). * **MIF (Migration Inhibitory Factor):** This was the first lymphokine discovered; it prevents macrophages from leaving the site of an antigen-antibody reaction. * **Key Trigger:** T cell activation and lymphokine release are the basis of **Type IV (Delayed-type) Hypersensitivity** reactions (e.g., Mantoux test).

Q: What are the products formed by the action of papain on an IgG molecule?

One Fc fragment and two Fab fragments. ### Explanation The structure of an Immunoglobulin (IgG) molecule can be analyzed by enzymatic digestion, a high-yield concept for NEET-PG. **1. Why Option B is Correct:** The enzyme **Papain** (derived from papaya) cleaves the IgG molecule **above the hinge region** (on the N-terminal side of the inter-chain disulfide bonds). Because it cuts above the point where the two heavy chains are linked, the molecule splits into three separate pieces: * **Two Fab fragments** (Fragment Antigen Binding): Each contains one entire light chain and the $V_H$ and $C_H1$ domains of the heavy chain. These are monovalent and can bind antigens but cannot precipitate them. * **One Fc fragment** (Fragment Crystallizable): Composed of the remaining $C_H2$ and $C_H3$ domains. This fragment is responsible for effector functions like complement fixation and opsonization. **2. Why Other Options are Incorrect:** * **Option A & C:** These are structurally impossible. An IgG molecule consists of only two heavy chains; therefore, it can only yield one Fc fragment (the joined "tail" of the two heavy chains). * **Option D:** This would imply the molecule was split vertically down the middle, which does not occur with enzymatic digestion. **3. Clinical Pearls & High-Yield Facts:** * **Pepsin Digestion:** Unlike Papain, Pepsin cleaves **below the hinge region**. This results in **one $F(ab')_2$ fragment** (bivalent, can precipitate antigens) and several small peptides (the Fc portion is degraded). * **Mercaptoethanol:** This is a reducing agent that breaks disulfide bonds, splitting IgG into **four separate polypeptide chains** (2 Heavy and 2 Light). * **Memory Aid:** **Pa**pain gives **Pa**rtial fragments (3 pieces); **Pe**psin gives a **P**recious bivalent fragment (1 large $F(ab')_2$).

Question 1

Neonatal thymectomy leads to which of the following changes?

Accepted Answer

Decreased size of paracortical areas

Answer

Decreased size of germinal centers

Answer

Increased antibody production by B cells

Answer

Increased bone marrow production of lymphocytes

Question 2

Immunoglobulin is used for all of the following diseases except?

Accepted Answer

Typhoid

Answer

Measles

Answer

Rabies

Answer

Chickenpox

Question 3

During the infancy of cardiac transplantation, nonhuman primate hearts were transplanted into humans to save lives. This type of transplantation is best described by which one of the following?

Accepted Answer

Xenograft: transplant from one species to another species

Answer

Allograft: transplant from one individual to another of the same species

Answer

Autograft: transplant from one region of a person to another region

Answer

Isograft: transplant from one person to a genetically identical person

Question 4

The opsonic index is the ratio of which of the following?

Accepted Answer

Phagocytic activity of a patient to the phagocytic activity of a normal individual

Answer

C3b opsonin to antibody opsonin

Answer

Serum concentration of opsonin in a patient to the serum concentration in a normal individual

Answer

Complement activity in a patient to complement activity in a normal individual

Question 5

Which complex of the complement system is known as the Membrane Attack Complex (MAC)?

Accepted Answer

C5-9

Answer

C3a

Answer

C3b

Answer

C4b

Question 6

Which of the following molecular phenomena in Ig genes is responsible for affinity maturation of antibody response?

Accepted Answer

Somatic hypermutation

Answer

Chain shuffling

Answer

Junctional diversity

Answer

Altered RAA splicing

Question 7

A 2-year-old boy presents with recurrent bacterial infections and Pneumocystis jiroveci pneumonia. Laboratory findings show markedly elevated IgM and low IgG. Which of the following is the most likely cause of his illness?

Accepted Answer

Deficiency in CD40 ligand or CD40

Answer

Autosomal-dominant mutation in the STAT3 gene

Answer

Defect in the NADPH oxidative pathway

Answer

Defective cytokine signaling in T-cell precursors

Question 8

Which of the following components of the immune system is responsible for the secretion of lymphokines?

Accepted Answer

T lymphocytes

Answer

B lymphocytes

Answer

Eosinophils

Answer

Macrophages

Question 9

What are the products formed by the action of papain on an IgG molecule?

Accepted Answer

One Fc fragment and two Fab fragments

Answer

Two Fc fragments and one Fab fragment

Answer

Two Fc fragments and two Fab fragments

Answer

One Fc fragment and one Fab fragment

Question 10

All of the following statements about carbohydrate antigen are true except?

Accepted Answer

Memory response is seen.

Answer

It has lower immunogenicity.

Answer

Causes polyclonal B cell stimulation.

Answer

Does not require stimulation by T cells.

Immunology — MCQs

Immunology — MCQs

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